Conventionally, an image forming apparatus includes a plurality of developing devices each accommodating toner of a different color, a plurality of photosensitive drums to which each toner is supplied, a conveyor belt for conveying each sheet, and a plurality of transferring devices for transferring each toner to each sheet. The plurality of photosensitive drums is juxtaposed and each sheet is printed in color when passing between each photosensitive drum and the belt unit.
In such a conventional image forming apparatus, for an easy maintenance, each photosensitive drum is supported in a photosensitive drum unit as an integral unit, and the photosensitive drum unit itself is configured to be detachably mountable in the image forming device. Also, a cleaning unit has been provided for cleaning toner and paper dusts deposited on the conveyor belt. Conventionally, the photosensitive drum unit is disposed upward of the conveyor belt, while the cleaning unit is disposed below the conveyor belt.